When Is My Next Period?
Enter your last period date for instant predictions. Ovulation, fertile window, and cycle phases β all calculated in your browser.
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How Does This Period Calculator Work?
This free period calculator predicts your next menstrual period based on the length of your average menstrual cycle. When you enter your last period start date and your typical cycle length, the tool calculates when your next period is expected, when you'll likely ovulate, and your fertile window β all instantly in your browser. No data is stored or sent to any server. Your privacy is absolute.
What Is a Normal Menstrual Cycle Length?
According to the American College of Obstetricians and Gynecologists (ACOG), a normal menstrual cycle ranges from 21 to 35 days, with an average of 28 days. However, only about 13% of women actually have a textbook 28-day cycle. Most women experience some variation month to month β and that's completely normal. Cycles can be affected by stress, sleep, exercise, weight changes, travel, and hormonal fluctuations. If your cycle is consistently shorter than 21 days or longer than 35 days, consider discussing it with your healthcare provider.
How to Calculate Your Period Manually
To calculate your next period manually: note the first day of your last period (Day 1), then count forward by your average cycle length. For example, if your last period started on March 1st and your cycle is typically 30 days, your next period would be expected around March 31st. Ovulation typically occurs about 14 days before your next period β so in this example, around March 17th. Your fertile window spans approximately 5 days before ovulation through 1 day after. This method works best for regular cycles. For irregular cycles, AI-based prediction tools like Cyra learn your personal pattern over time for greater accuracy.
Why AI Period Prediction Is More Accurate
Traditional period calculators use simple arithmetic β they assume your cycle length is fixed. But research published in NPJ Digital Medicine (Bull et al., 2019) analyzing over 600,000 menstrual cycles found significant variation in cycle length, even among women who consider their periods "regular." AI-based prediction, like what the Cyra app uses, doesn't assume a fixed cycle. Instead, it learns from your personal data β analyzing patterns in your cycle lengths, the relationship between your cycles and factors like stress and sleep, and improving its predictions with each cycle you log. Studies suggest AI-based methods can be 2-3 days more accurate than calendar-based prediction.
When to See a Doctor About Your Cycle
While some cycle variation is normal, certain patterns warrant medical attention. See a healthcare provider if: your periods are consistently shorter than 21 days or longer than 35 days; you miss three or more consecutive periods (when not pregnant); your periods are extremely heavy (soaking through a pad every hour); you experience severe pain that interferes with daily activities; or you notice significant changes in your usual cycle pattern. These symptoms may indicate treatable conditions such as PCOS, thyroid disorders, or endometriosis. Early evaluation leads to better outcomes.
This calculator provides estimates based on average cycle mathematics. It is not a substitute for medical advice. For personalized predictions, use the Cyra app which learns from YOUR cycle data.